Digital circuitry incorporated into electronic apparatus sometimes enters an in an indeterminate state. For example, when power is applied to the circuitry, either at start-up time, or after a power outage, the power-on state of the digital circuitry is unknown. Also, during a power brown out, the lowered supply voltages may cause digital circuitry to enter an unknown state. In addition, during operation of the apparatus containing the digital circuitry, transient signals (which are short duration, high frequency signals, possibly with high voltage), induced either on power supply lines, signal lines or within the digital circuitry itself, may cause the digital circuitry to enter an unknown state. In such situations, it is necessary to provide a reset signal which forces the digital circuitry into a predetermined state, from which proper operation may be resumed.
In a power-on condition, the power supply voltages are just ramping up, and the digital circuitry is not yet operating. Under these conditions, some digital circuitry requires a reset signal having a longer duration than would be necessary under normal operating conditions. However, in the other two situations, (during a brown out and after a transient signal), the digital circuitry is operating, and the voltages are stable. In such situations, a reset signal having a normal duration is sufficient. Thus, the conditions under which reset signals are required are different, and the parameters of the required reset signals are correspondingly different.
In particular, detection of transients is difficult because of their high frequency content. Any capacitive reactance coupled to ground in the reset circuitry will tend to dissipate the high frequencies, and may cause a transient to go undetected. Thus, to detect transients, capacitance must be minimized in the detector circuit. On the other hand, the power-on-reset signal must be held for a long enough time period for the power supply voltages to reach their proper levels, and for the digital circuitry to begin proper operation. This generally has been done by using RC circuits with large time constants and, consequently, large capacitance values.
A reset circuit is desirable which provides a reset signal having a normal duration in response to conditions during powered operations of the electronic system requiring the reset signal, but having a modified (e.g. extended) duration during power-on conditions. A reset circuit is further desirable which provides for optimized detection of transients, while providing a long enough power-on-reset signal, and which does not require a large number of components, which increases cost, and adversely affects reliability.
In accordance with principles of the present invention, a reset circuit in an electronic system includes a reset signal generator which generates a reset signal in response to predetermined conditions in the electronic system. A power-on detector is provided to cause the reset signal generator to modify the reset signal generated in response to a power-on situation.